The series B1E, D4H, F9K, H16N goes on by a rule of its own. What is its fifth term?

The opening letters B 2, D 4, F 6, H 8 go on by two, so the next is 10, which is J. The numbers 1, 4, 9, 16 are the squares of 1, 2, 3 and 4, so the next is 25. The closing letters E 5, H 8, K 11, N 14 go on by three, so the next is 17, which is Q. That gives J25Q. Advancing the closing letter by only two gives J25P; adding 4 to the number instead of taking the next square gives J20Q; advancing the opening letter by three gives K25Q.